A global navigation satellite system (GNSS) is a system of space-based satellites that provides autonomous geo-spatial positioning with global coverage. Generally, a GNSS allows receivers to determine their location using time signals transmitted along a line-of-sight from the satellites. The Global Positioning System (GPS) is a GNSS that is maintained by the United States government and can be used by anyone with a GPS receiver. Similarly, GLONASS is a navigation satellite system maintained by Russia. The Galileo system is another GNSS that is currently being built by the European Union (EU) and European Space Agency (ESA). COMPASS is a navigation satellite system being developed by China.
A GNSS provides location information anywhere on or near the Earth where there is an unobstructed line of sight to four or more GNSS satellites (assuming synchronized time among the particular GNSS constellations). A processor coupled to the GNSS receiver uses at least four of the distances from the receiver to the satellites, known as pseudoranges, to accurately estimate the position of the receiver. The accuracy of the estimated position, or position solution, varies as changing atmospheric conditions affect signal-to-noise ratios and signal transit times. The accuracy also varies as the orbiting satellites occasionally experience protracted failures during which they continue to operate while providing erroneous or extra-noisy signals. These and other factors appear as random noise in the transmitted signals, random errors in the computed pseudoranges, and ultimately as a random error in the position solution itself.